1. Field of the Invention
This invention relates to an evaporative emission control system for internal combustion engines, which has a self-diagnosis function of determining whether the system is normal.
2. Prior Art
An evaporative emission control system is conventionally provided for internal combustion engines, which is generally constructed such that evaporative fuel generated in a fuel tank is adsorbed in a canister for temporary storage therein, and the stored evaporative fuel is fed into the intake pipe at a location downstream of a throttle valve in the intake pipe when the engine is in a predetermined operating condition. An evaporative emission control system of this kind has been proposed e.g. by Japanese Laid-Open Utility Model Publication (Kokai) No. 2-26754, which includes a purge control valve arranged in a purging passage connecting between the canister and the intake pipe and a vacuum switch arranged in the purging passage at a location between the purge control valve and the canister. The proposed emission control system has a self-diagnosis function of determining whether the system is normal, i.e. whether purging of evaporative fuel is being normally carried out (this determination will be referred to as "the purge flow check") based on a change in the output from the vacuum switch when the purge control valve is opened and closed.
However, the proposed system is disadvantageous in that its manufacturing cost is increased due to the employment of the vacuum switch and it is incapable of accurately carrying out the purge flow check if the vacuum switch per se has low reliability.